Refractometer

ABSTRACT

There is disclosed a critical-angle hand refractometer for measuring the refractive index of an unknown substance by the position of the edge of a shadow (optical image). The refractometer comprises refracting means to linearize the refractometer scale and temperature compensating means which effect a temperature compensation by a rectilinear displacement of the scale or the objective lens of the refractometer without altering the magnification of the optical system of the refractometer.

United States Patent Inventor Herbert E. Goldberg Concord, Mass.

App]. No. 66,572

Filed Aug. 24, 1970 Patented Dec. 7, 1971 Assignee American OpticalCorporation Southbridge, Mass.

Continuation of application Ser. No. 704,309, Feb. 9, 1968. Thisapplication Aug. 24, 1970, Ser. No. 66,572

REFRACTOMETER 16 Claims, 15 Drawing Figs.

US. Cl 356/135 G01n 21/46 Field of Search 356/135, 128

References Cited UNITED STATES PATENTS 2,619,003 11/1952 Polanyiu356/135 3,006,241 10/1961 Marks et a1. 350/193 X 3,267,795 8/1966Goldberg... 356/137 3,279,309 10/1966 Goldberg... 356/135 3,329,0607/1967 Holleran 356/135 FOREIGN PATENTS 604,469 7/1948 Great Britain356/135 Primary Examiner-Ronald L. Wibert Assistant Examiner-T. MajorAttorneys-William C. Nealon, Noble S. Williams, Robert].

Bird and Bernard L. Sweeney ABSTRACT: There is disclosed acritical-angle hand refractometer for measuring the refractive index ofan unknown substance by the position of the edge of a shadow (opticalimage). The refractometer comprises refracting means to linearize therefractometer scale and temperature compensating means which effect atemperature compensation by a rectilinear displacement of the scale orthe objective lens of the refractometer without altering themagnification of the optical system of the refractometer.

' PATENTED DEB H971 v sum 1 0F 2 ERROR 0F REA /N6 PERCENT sucRos 6 20TEMPERATURE FIG. 2

ESGQLfT/ON I u:

FIG. 4a

(FL/N, 5

CROW/V FIG. 5 v

ESCALAT/ON PERCENT SUCROSE FIG. 3

PERCENT SUCROSE FIG. 4b

FIG. 6

I NVENTOR.

HERBERT E, GOLDBERG A rroku: rs

ERROR F R'ADM/G PERCENT SUCROSE' PATENTEU [IE8 I an NIAGN/F/CAT/ON us'50 41s ANGLE OF INCIDENCE 1',

FIG. 7

CONVE/VT/OML SINGLE RISK PRISM AA-IGLE A 'coA/veurlaml. swan: PRISM ANDl0 HEDGE FIG. 8

CONVENTIONAL SINGLE m/sM TEMPER TURE "c FIG;

$CALATION FIG.

FREfiZ/NG POINT 6 FIG. I I

I/[RBERT E. GOLDBERG I N VENTOR.

ArroRMsYs REFRACTOMETER This application is a continuation of mycopending application, Ser. No. 704,309, filed Feb. 9, 1968.

This invention is concerned with systems of correcting the temperatureerror of refractometers, particularly of so-called critical-angle handrefractometers. It is also concerned with means to modify thenonlinearity of refractometer scales in order to improve the accuracy ofreading at the low portion of the scale and to increase the range ofreading of the instrument.

Critical-angle refractometers are instruments with which the refractiveindex or related characteristics of unknown substances may be measuredby reading on a divided scale the position of the edge of a shadow,sometimes called the optical image. The principle of operation of suchrefractometers is described in detail in textbooks and in the patentliterature, e.g., in U.S. Pat Nos. 2,619,003, 3,267,795, 3,279,309,which also relate to the temperature error of refractometricmeasurements and ways of correcting it. It has been stated in thesepatents that the rate of displacement of the optical image, which occursas the temperature of measurement changes, is not the same at all pointsof the scale. Because of this, it has been thought impossible heretoforeto correct the temperature error by a simple displacement of therefractometer scale with respect to the optical image-except in the caseof refractometers having unusually narrow ranges of measurement.

The systems, which have been proposed in the aforementioned patents tocorrect the temperature error, comprise devices for adjusting either themagnification of the refractometer or the effective size of the scale inaccordance with temperature changes. The system described in U.S. Pat.No. 3,267,795 employs a fixed compensator prism cut of a specialmaterial which changes refractive index as a function of temperature.Since the optical effect produced by a given index change depends on theangle of incidence of the light beam forming the optical image, propercompensation may be obtained at several points of the refractometerscale by selecting the orientation of the compensator as explained inthe aforementioned patent. The variable index system of temperaturecompensation does not comprise moving elements. It is very stable and iswidely used in the industry, even though variable index prisms arecostly to produce.

Another system described in U.S. Pat. No. 3,279,309 relies on acompensator lens or prism which is rotated about a transverse axis by athermally responsive actuator. The optical effect caused by a givenrotation again depends on the angle of incidence, and correctcompensation of several points of the scale is again obtained by properorientation of the compensator in the optical path, which in this case,however, changes with temperature. Since the orientation of the prismalso con trols the overallmagnification ofthe optical system, it must beadjusted carefully at the factory. If it is not set properly, or if itchanges in the course of time, large errors of reading will appear atthe high end of the scale, even if the setting of the zero point iscorrect. The moving compensator type of instrument must, hence, bechecked periodically and adjusted if necessary, using standard solutionsof both low and high index.

The need for periodic checks of the reading at the high end of the scaleis a serious disadvantage of the moving prism compensator as comparedwith the variable index type because standard aqueous solutions of highindex are likely to change with time. Test prisms, on the other hand,must be used at certain well defined temperatures sincetheir temperaturecoefficients are always much lower than those for which the compensationsystem of the refractometer was designed.

If either the high or the low reading is faulty, the compensatororientation as well as the position of the scale with respect to theimage must be reset. Since both adjustments are inter dependent,adjustment is very difficult to perform unless specialized test fixturesare available. It is for this reason that the moving prism type ofcompensators has been restricted to industrial refractometers requiringonly a limited accuracy of reading.

It is a broad object of the present invention to provide asystem oftemperature compensation which employs a simple rectilinear displacementof the refractometer scale, or of the optical image. It thus functionswithout altering the magnification of the optical system. Such systemhas the advantage of the variable index system in that the entireinstrument calibration is detennined by the setting of the zero point ofthe scale and can be verified by checking it on any other single point.It is, however, much more flexible in design, less costly to construct,and perhaps sturdier in use.

Other objects, features and advantages of the invention will be pointedout hereinafter and set forth in the appended claims constituting partof the application.

In the accompanying drawing several embodiments of the invention areshown by way of illustration and not by way of limitation.

In the drawing:

FIG. 1 is a schematic view of the optical system of a criticalanglerefractometer according to one form of the invention;

FIG. 2 shows the temperature error which is committed when a 10 percentsucrose solution is read with a conventional uncompensatedrefractometer;

FIG. 3 illustrates the degree of temperature compensation of the sucroserefractometer obtainable with various optical systems;

FIGS. 4a, 4b and 4c illustrate the reduced nonlinearity of refractometerscales depending on the design of the measuring prism;

FIG. 5 illustrates a composite measuring prism of large prism angle,designed to be achromatized at three points of the scale;

FIG. 6 shows a dual inclined wedge optical system for reducing thenonlinearity of the refractometer scale;

FIGS. 7 and 8 illustrate optical characteristics of inclined wedgesystems;

FIGS. 9a, 9b and show the residual temperature errors of sucroserefractometers with various optical systems;

FIG. 10 shows a type of the refractometer scale which is useful in manypractical applications; and

FIG. I1 is a chart similar to FIG. 3 but designed to illustrate the typeof temperature compensation obtainable for an ethylene glycolrefractometer.

Referring now to FIG. 1, it is not considered necessary to explain herethe path of the light rays and the way the optical image is formed andmay be observed through an eyepiece 7, since this is well known to thoseskilled in refractometry. Attention is drawn to the fact that themeasuring prism 2 and the magnitude of the refracting angle a of thisprism are entirely conventional. The substance to be measured isindicated at 9. A cover plate 10 is preferably placed upon thesubstance. It is customary in the design of hand refractometers toselect angle a so that the refraction at the exit face 8 is in theopposite direction from that taking place at the entrance face I, so asto compensate all or most of the color dispersion which occurs at theentrance face, at least for the midpoint of the refractometer scale.Accordingly, the exact value of angle a depends on the dispersion of thesolutions to be measured and on the type of glass of which the measuringprism is cut; it is usually between 30 and 60.

The purpose of a stationary refracting means shown as a fixed prism orwedge 3, on the other hand, is not conventional, but is an element ofone embodiment of the invention.

Exit face 8 of prism 2 may also function in a similar way as astationary refracting means under certain conditions as will beexplained more fully hereinafter.

A thermal actuator 5 also forms part of the invention as it pertains totemperature compensation. As shown, it causes displacement between theoptical image and the reticle 6 by raising or lowering the objectivelens 4 in accordance with temperature changes. The reticle may carry ascale 6a with a base line 6b, constituting a reference mark as shown inFIG. 10. In an alternate but equivalent embodiment of the invention, itmay displace the reticle 6 instead of the objective lens. In otherembodiments of the invention, a thermal activator may cause the requireddisplacement in other well-known ways, for instance, by rotation of aplane parallel plate positioned in the light path.

The functioning of temperature compensators according to the inventionmay be clarified by introducing the concept of escalation."Suppose thatthe scale or reticle of the refractometer reads in units of q where qmay be the refractive index, percent sucrose, degrees freezing pointdepression, or any other quantity to be measured. Suppose further that sis the distance between the point of reading q and the point q, at thebase line of the scale, s=f(q). The escalation" of the spacing ds/dq ofthe refractometer scale divisions between points q and q, is representedby (e )q,q0:

dlldq)q,qo The escalation of the scale measures its departure fromlinearity. it is larger than unity for expanding scales and smaller forthe compressed type. The nonlinearity at any point of the scale may alsobe measured by the second derivative d,.r/dq prevailing at that point.

Similarly, one may define the escalation of the temperature change dq/d!of the reading q as dqIdl t1,Qo q/ 0 q/dt )qu and the escalation of themotion of the refractometer shadow with temperature ds/dt 01m) )ooTemperature compensation may be obtained at two points, q and of thescale by a straight line displacement of the scale with respect to theimage, if ds/dt is the same at these two points.

An inclined auxiliary prism or wedge such as wedge 3 may be positionedbetween the measuring prism 2 and the refractometer scale 6 to introducean escalation factor smaller than unity and thus to reduce the totalsystem escalation. For instruments covering unusually broad ranges ofmeasurement, a combination of both methods may be employed.

Since dnldF duIdq dqMr We have by substitution in la):

( ur/ac) X du/dt) 1 ("til/tin) du/dt) qo Equation (3) means that theescalation of the refractometer scale must be inversely proportional tothat of the temperature coefficient of the materials to be measured. Thelast column of table 1 lists typical values of the scale escalationrequired to satisfy equation (3) for a sucrose refractometer.

The data shown later in this specification demonstrate that the sucrosescale escalations listed in column 4 of table I are obtainable forinstance by combining a measuring prism 2 of 60 large refracting angle aand escalation of 1.2 with an inclined fixed wedge 3 of escalation 0.75.Actually, such an elaborate system is non needed, because conditionsencountered in practice are less severe. Indeed, the data given inmanufacturers catalogues show that all commercially available systems oftemperature compensation produce perfect results only at two points ofthe scale, and that small residual errors remain at all other points. Ifsimilar errors were permitted in the system disclosed herein, thecompensation required at various scale points would not have to matchprecisely the temperature error of the solution at those points, butcould be allowed to vary between certain limits, defined for each pointby:

4. (dq/d!) compensation (dq/dl) mat. (dq/d!) allowable error where mat."is material to be measured.

allowable orrorL,

( dq/dt) c0mpensation= [(dq/dt)mat. (dq/dt) allowable error],compensator[(dq/dt)mat.

:l: (dq/dt) allowable error] Similarly, if the negative error sign isused in the numerator, limits of allowable scale escalation are foundfor the weakest possible compensator. The numerator being smaller inthis case, the allowable scale escalation will be smaller also.

[(dq/dt)mat. (dq/dt) allowable errorl compensator [(dq/dt)mat.

:l: (dq/dl) allowable error] ds/d g) most active:

(7) (e least active The numerical data, which follow, will demonstratethat the concept expressed by equation (6) reduces demands on escalationsignificantly. The sucrose refractometer will be considered as a firstexample, because it is the standard laboratory instrument for themeasurement of "soluble solids. Since the temperature changes in therefractive index are almost entirely due to thermal expansion of thesolvent, the figures are valid not just for sucrose, but for aqueoussolutions of most inorganic compounds as well.

As a second example, an ethylene glycol refractometer will be analyzedin order to demonstrate the advantages of the invention for industrialinstruments which require limited accuracy but must function over awider range of temperature changes.

1. Sucrose Refractometers Subsequent table 2 gives estimates of residualtemperature errors of sucrose refractometers allowable for laboratoryand for industrial use. The table also lists the temperaturecoefficients dq/dt for several concentrations of solution, and themaximum and minimum dq/d! which the compensator must supply at eachconcentration to keep the residual errors within the specified limits.Since the temperature coefficient of sucrose solutions below 25 percentis a function of temperature as well as concentration, equation (4)cannot be used in these cases. (dq/d!) must then be found by thegraphical method illustrated in FIG. 2. In this FlG., the temperatureerror of reading has been plotted against temperature for a 10 percentsucrose solution. The slope of the curve is therefore equal to (dq/d!)solution. Two straight lines have been drawn, having the highest andlowest slope possible without departing from the curve by more than theallowable error. The slope of these lines is (dq/dUmatIMq/d!) allowableerror and has been entered in columns 5 and 6 ofTable 2.

( 'l -)Compcnntcr for laboratory service. -l -)Compcnuntor forindustrial service. Allowable escalation for laboratory service.

8 Allowable escalation for industrial service.

Percent C. eds./dq.

Percent Percent C. Max. Min. Max. Min. Max. Min. Max. .\lin.

TABLE 2-Continued Column: Quantity shown 1 Concentration. 2. (dg-/dlnMnterlal- 3.. Allowable error for laboratory service. 4 Allowable errorfor industrial service. 5.. (GgJdtJ Om MM, for laboratory service. 6..(dgJdtJ for industrial service.

Allowable escalation for laboratory service. 8 Allowable escalation forindustrial service.

Percent C. eds./dq.

Percent Percent C. Max. Min. Max. Min. Max. Min. Max. Min.

The data shown in columns 7 and 8 of the table have been calculated fromequation (6) and plotted in FIG. 3. The

curves 0A, OB and OC of that FIG. delineate the ranges of the overallinstrument scale escalation which permit temperature compensation bysimple displacement between the scale and the image.

The area AOC determines escalation for industrial service, while themore restricted contour BOC applies to laboratory service.

Similar contours could be calculated according to equation (7). Theywould applyto the least active compensators that can be used. They areof little practical interest, however, because of the heavy demands theymake on escalation.

The escalation curve of a conventional hand refractometer is representedby curve I in FIG. 3. This curve applies to all such instruments becauseescalation does not depend greatly on the refractive index of themeasuring prism. The reason is that measuring prisms of high refractiveindex also have higher dispersion. They generate less escalation of theentrance face, but increased chromatic aberration, which must becompensated by increased reverse refraction of the exit face. This inturn brings total escalation to the level characteristic of low indexprisms.

FIG. 3 demonstrates that the temperature error of conventional handrefractometers can be compensated by simple displacement of the scalewith respect to the image for a sucrose range from 0-5 percent in caseof laboratory service, and up to 15 percent for industrial uses. Theseranges clearly are too narrow. Escalation must be reduced in order toextend them, and this may be accomplished by inserting the specialstationary refracting means as discussed before into the system.

FIGS. 4a, 4b and 4c show escalation curves for sucrose refractometers,employing measuring prisms of large refracting angles. Data are includedfor refractive indices of n=1.52, n=l.62 and n=1.75. Several prismangles a were chosen for each index. beginning with the angle that wouldreduce the escalation to 1.2 for the 0=30 percent range, and going tothe largest angle that would limit the angle of refraction at the exitface to 75.

The 45-curve in FIG. 4a represents a conventional hand refractometerprism and has been included for comparison.

Prisms of large refracting angles generate, of course, substantialchromatic aberration. They must be achromatized, for instance, bysplitting them into Flint-Crown assemblies according to FIG. 5 whichshows such a prism having a refracting angle a larger than 90. If theglasses are chosen such as to have the same index for the D line, aspherical interface 15 may be employed to achromatize the assembly atthree points of the refractometer scale without introducing astigmatism.

produced at the measuring face. The linearizing effect is 7 achieved bythe expansion of the low end of the scale. The field angle of theinstrument is, hence, increased and the telescope magnification neededfor proper viewing of the scale is reduced.

Escalation can be reduced also by utilizing the optical distortiongenerated by decentered lenses, separate additional prisms, or inclinedwedges. Consider the wedge B shown in FIG. 6. The angular deviation x(FIG. 6) suffered by a light ray passing through it depends on itsrefractive index and the refracting angle, as well as the angle ofincidence. Deviation is at a minimum when angles of incidence i and orrefraction r are equal. However, the magnification dr/di does not passthrough a minimum, but increases continuously from zero to infinity asangle 1', decreases from to the angle at which angle 1': reaches 90.FIG. 7 shows the magnification as a function of angle of incidence. Theportions of the curve which are of greatest interest for the presentpurpose correspond to angles of incidence comprised between 50 to 20 andbetween +40 and +75 respectively.

Positive angles of incidence correspond to the orientation of wedge A ofFIG. 6. The magnification is smaller than unity and decreases as theangle of incidence increases. Such a wedge could compress the high endof a refractometer scale and reduce the field angle. The configurationof prism B represents negative angles of incidence. In this casemagnification is above unity and the wedge acts to expand the low end ofa refractometer scale. The combination A and B shown in the FIG. willthen affect both ends of the range, reducing escalation without alteringthe field angle greatly. A similar result may be achieved by pairing awedge using positive angles of incidence with one of the measuringprisms of large refracting angle described previously.

FIG. 3 shows the escalation characteristics of wedge prisms as afunction of the prism angle b. Angles of incidence of +55 and +70 havebeen used in this example to represent a typical 0-50 percent sucroserefractometer having a field angle of 15.

It is seen from the FIG. that escalation does not change greatly afterthe wedge p'rism angle exceeds 10 or 15. The curve has been drawn for awedge prism index of 1.62, but two points representing indices of 1.52and 1.75 have been included for the IO-degree prism angle to demonstratethat the wedge escalation is nearly independent of index. This is animportant point, because it allows the designer to minimize chromaticaberration by employing glasses of low dispersion.

Calculations show, for instance, that a 10 wedge made of 1.52/64 glassproduces about 2 minutes of chromatic aberration for a 60angle ofincidence. Such a color band can be corrected easily, either byadjusting the refracting angle a of the measuring prism, or by followingthe procedure described in US. Pat. No. 3,267,795. This involves the useof a decentered objective lens generating 0. of paraxial and 0.015 oflateral color.

The considerations expressed above may be summarized by suggesting thefollowing designs for sucrose refractometers of reduced escalation:

TABLE 3 Range percent sucrose -20 0-50 0-75 Measuring prism CompositeSingle Composite. Flint glass and angle 1.517/52, 16 None 1.755/77,Crown glass and angle 1.517/70, +112 1.57/56, +41% 1.755137 0 InterfaceFlat Curved. Wedge glass and angle- 1.520/64, 10- None. Wedge incidencefor 0% bea -44 or +63 Residual color :l:10 O.14:l:0.01 Field angle 90 or6.3 23. Objective lens 1.7 27 prism ang e 80 mm./none 9O mm./4.7 prmn1./nonc. Step position in front of objective 8 mm 2 mm. or 6 mm Atcrown. Scale interval micron spacing) 1% 0.2% 0. Scale length 12 mm 10Eye piece magnificatiom Scale escalation Range Indus- Laboratrial toryCurve service, service, Number Type of prism system percent percent 11.617 conventional a=40 0-15 0-5 1. 517 flint-crown a=96 0-35 0-25 1.575single crown a=41 and 10 0-65 0-50 wedge. 4 1.755 flint-crown a=8l 0-760-75 The escalation curves 1, 2, 3 and 4 are positioned for the mostpart well within the areas ADC and BOC and touch the boundaries only atthe extreme end of the refractometer range, where the instrument isseldom used. For the great majority of applications the compensation is,hence, well within the tolerances specified in table 2. As an example,the actual performance of systems No. 1 and 3 have been computed andplotted in FIGS. 90 and 9b. it has been assumed that both systems havebeen fitted with a thermal actuator 5 according to FIG. 1, displacingthe scale by 0.076 percent sucrose division for each degree C.temperature change, measured near the zero point ofthe scale. Similarresults would be obtained with systems No. 2 and 4. All comparefavorably with error data of temperature compensation systems presentlycommercially available.

The performance of these systems can be improved further, however, if itis considered that pure water is never measured but serves only as amedium for calibration. Taking advantage of the fact that the dq/d!curve for water is a straight line above 25 C., the reticle could bedesigned with divisions between 0 and 7 percent omitted, and carrying anotation instructing the user to adjust the zero line at an instrumenttemperature comprised between 25 and 35 C. This implies noinconvenience, since water of this temperature will be availablewherever calibration work is done. FlG. 9c, for instance, indicates theexcellent results that may be obtained with the single prism instrumentif the reticle pattern shown in FIG. 10 is employed in conjunction withan actuator displacing the scale by 0.084 percent sucrose per degree C.temperature change, measured near the zero point of the scale.

2. Ethylene Glycol Refractometers The quantity measured in the ethyleneglycol (or glycol") refractometer is the freezing point of coolingsolutions used for instance in the radiators of internal combustionengines. The accuracy of measurement required in this application isonly moderate, perhaps of the order of 0.002 refractive index units, butthere is a great need for temperature compensation nevertheless, becauseof the widely varying climatic conditions under which the instrumentsare used. Ambient temperatures may range, for instance, from +35 C. downto the freezing point of the solution that is checked. For practicalpurposes, the range is somewhat more limited, because it may be assumedthat solutions of very low freezing point are not usually measuredduring the summer months, or above +20 C., and that 25 C. is the lowestinstrument temperature at which accurate readings can be taken, even inwinter time.

The temperature coefficient dq/d! of the quantity measured by the glycolrefractometer is the change in the freezing point, read in degrees C.,which is caused by changes of the temperature at which the reading istaken, also expressed in degrees C.

Table 4 lists the data which are required for an analysis of theproblem. Column 4 shows the temperature ranges at which glycol solutionsof various strengths may be assumed to be handled, and column 6 themaximum errors permitted at the extremes of these ranges. Column 5 givesthe temperatures at which the position of the respective scale pointsshould be calculated, so that there will be no error at thosetemperatures. Measurements may, of course, be made at temperaturesbeyond the limits set in column 4, but at a penalty of larger errors.

The last column of the table shows the errors per degree temperaturechange, that would be allowable within the specifications set in columns4 and 6. All temperatures and errors are in degrees C.

The action of compensator which is required to reduce the temperatureerror to the limits set in table 4 is determined again by equation (4).

The limits within which compensation must be generated are shown intable 5. column 4. The maximum and minimum permissible escalations of(dq/do which results, are listed in column 5. They have been computedaccording to equation (5) with reference to the -7 point of the scale,rather than the zero point, in order to avoid the ambiguities whichwould otherwise be created by the fact that no compensation at all, oreven negative compensation, is acceptable at the zero point of theparticular instrument.

TABLE 5 Column: Quantity shown 1 Freezing point. 2 (dg./dl-)Bolutinn-3-. Allowable error per degree temperature change. 4 Compensationrequired. 5 Escalation of the required compensation calculated relativeto the 7 0. scale point. 6 Allowable scale escalation relative to the 70. scale point.

Most active Least active 0.090, C./ 0.040, C./ C./ C. comp. 0.090 comp.0.040 C. comp. C. comp. C. Max Min Max. Min. Max. Min. Max. Min. Max.Min.

The scale escalations ds/dt, which are shown in column 6, jective havingan image plane, a reticle disposed in said image have been calculatedaccording to equation (6) and have been plane and carrying a scalehaving a reference mark, said enplotted in FIG. 11. Any refractometerwith an escalation curve that lies entirely within the area ABCD, willbe compensated against temperature changes within the specificationsgiven in Table 4, if the compensator has an activity of dg/dt=0.090 C./C. at the 7 C. point of the scale. This is the highest activitypossible. The escalation curve for a refractometer system composed of a41 single prism and a 10 linearizing wedge has been entered in dottedoutline. It is seen that this system meets the specifications set intable 4. it would perform even better if the tolerance area ABCD wereshifted downward slightly by recomputing the data of columns 5 and 6 oftable 5 for a slightly less active compensator with an activity at the 7C. point of 0.08 C. freezing point reading per degree C. temperaturechange.

What is claimed is:

l. A refractometer operating with a source emitting bundles of light andhaving a measuring prism with an entrance face to receive differentsubstances within a range of characteristics to be measuredcorresponding to refractive indices n, an objective having an imageplane, a reticle disposed in said image plane and carrying a scalehaving a reference mark, said entrance face and said reticle beingpositioned in such a way that bundles of light from said source arerefracted and deviated at said entrance face and directed to saidobjective, and focused by said objective to an optical image on saidscale at a distance .r from said reference mark, ds/dn being the rate ofchange in said distance 5 with said refractive index n, and d sldn beingthe nonlinearity of said rate of change in combination with:

stationary refracting means positioned between said entrance face andsaid reticle, said stationary refracting means deviating said bundles oflight by angles of deviation .t, dx/dn being the rate of change of saidangles of deviation with said refractive index n to be measured, saidstationary refracting means being oriented so that dx/dn decreases as nincreases thereby causing a reduction of said nonlinearity d s/dn saidstationary refracting means being constituted by an exit face of saidmeasuring prism, said exit face producing a deviation of said bundles oflight in the same direction as the deviation of the bundles of light atsaid entrance face.

2. A refractometer according to claim 1 wherein said measuring prism hasa refractive index below 1.62 and a prism refracting angle larger than90.

3. A refractometer according to claim 1 wherein said measuring prism hasa refractive index above L62 and a prism refracting angle larger than75.

4. A refractometer according to claim 1 wherein said measuring prism iscomposed of at least two components having different optical dispersion.

5. A refractometer according to claim 4 wherein said components havesubstantially the same refractive index for one line of the spectrum andare separated by curved faces.

6. A refractometer operating with a source emitting bundles of light andhaving a measuring prism with an entrance face to receive differentsubstances within a range of characteristics to be measuredcorresponding to refractive indices n, an obtrance face and said reticlebeing positioned in such a way that bundles of light from said sourceare refracted and deviated at said entrance face and directed to saidobjective, and focused by said objective to an optical image on saidscale at a distance s from said reference mark, ds/dn being the rate ofchange of said distance s with said refractive index n. and d r/dn beingthe nonlinearity of said rate of change in combination with;

stationary refracting means positioned between said entrance face andsaid reticle, said stationary refracting means deviating said bundles oflight by angles of deviation x, dx/dn being the rate of change of saidangles of deviation with said refractive index n to be measured, saidstationary refracting means being oriented so that dx/dn decreases as nincreases thereby causing a reduction of said nonlinearity d s/dn saidstationary refracting means including a fixed prism having refractingfaces and being positioned between said measuring prism and saidreticle.

7. A refractometer according to claim 6 wherein said fixed prism isoriented so that the angle of incidence of said bundles of light at atleast one of said faces is positive and comprised between 40 and 75.

8. A refractometer according to claim 6 wherein said fixed prism isoriented so that the angle of incidence of said bundles of light at atleast one of said faces is negative and comprised between 20 and 50.

9. A refractometer operating with a source emitting bundles of light andhaving a measuring prism with an entrance face to receive differentsubstances within a range of characteristics to be measuredcorresponding to refractive indices n, said refractive indices varyingwith temperature changes, an objective having an image plane, a'reticledisposed in said image plane and carrying a reference mark, saidentrance face and said reticle being positioned in such a way thatbundles of light from said source are refracted and deviated at saidentrance face and directed to said objective, and focused by saidobjective to an optical image on said reticle at a distance s from saidreference mark, ds/dn being the rate of change of said distance s withsaid refractive index n, and d s/dn being the nonlinearity of said rateof change in combination with:

temperature compensating means compensating for the temperature errorresulting from the temperature coefficient of the refractive index ofsaid different substances to be measured, said temperature compensatingmeans comprising a thermal actuator to move one of said objective andsaid reticle so as to cause displacement between said optical image andsaid reference mark in accordance with temperature changes, andstationary refracting means positioned between said entrance face andsaid reticle, said stationary refracting means deviating said bundles oflight by angles of deviation is, dx/dn being the rate of change of saidangles of deviation with said refractive index n to be measured, andbeing oriented so that dx/dn decreases as n increases thereby causing areduction of said nonlinearity d s/dn the stationary refracting meansbeing constituted by an exit face of said measuring prism, said exitface producing a deviation of said bundles of light in the samedirection as the deviation of the bundles of light at said entranceface.

10. A refractometer according to claim 9 wherein said measuring prismhas a refractive index below 1.62 and a prism refracting angle largerthan 90.

11. A refractor according to claim 9 wherein said measuring prism has arefractive index above 1.62 and a prism refracting angle larger than 75.

12. A refractometer according to claim 9 wherein said measuring prism iscomposed of at least two components having different optical dispersion.

13. A refractometer according to claim 9 wherein said components havesubstantially the same refractive index for one line of the spectrum andare separated by curved faces.

14. A refractometer operating with a source emitting bundles of lightand having a measuring prism with an entrance face to receive differentsubstances within a range of characteristics to be measuredcorresponding to refractive index n, said refractive indices varyingwith temperature changes, an objective having an image plane, a reticledisposed in said image plane and carrying a reference mark, saidentrance face and said reticle being positioned in such a way thatbundles of light from said source are refracted and deviated at saidentrance face and directed to said objective, and focused by saidobjective to an optical image on said reticle at a distance s from saidreference mark, ds/dn being the rate of change of said distance s withsaid refractive index n, and d sldn being the nonlinearity of said rateof change in combination with:

temperature compensating means compensating for the temperature errorresulting from the temperature coefficient of the refractive index ofsaid different substances to be measured, said temperature compensatingmeans comprising a thermal actuator to move one of said objective andsaid reticle so as to cause displacement between said optical image andsaid reference mark in accordance with temperature changes, andstationary refracting means positioned between said entrance face andsaid reticle, said stationary means deviating said bundles of light byangles of deviation x, dx/dn being the rate of change of said angles ofdeviation with said refractive index n to be measured, and beingoriented so that dx/dn decreases as n increases thereby causing areduction of said nonlinearity d sldn the stationary refracting meansincluding a fixed prism having refracting faces and being positionedbetween said measuring prism and said reticle.

15. A refractometer according to claim 14 wherein said fixed prism isoriented so that the angle of incidence of said bundles of light at atleast one of said faces is positive and comprised between 40 and 75.

16. A refractometer according to claim 14 wherein said fixed prism isoriented so that the angle of incidence of said bundles of light at atleast one of said faces is negative and comprised between 20 and 50.

1. A refractometer operating with a source emitting bundles of light and having a measuring prism with an entrance face to receive different substances within a range of characteristics to be measured corresponding to refractive indices n, an objective having an image plane, a reticle disposed in said image plane and carrying a scale having a reference mark, said entrance face and said reticle being positioned in such a way that bundles of light from said source are refracted and deviated at said entrance face and directed to said objective, and focused by said objective to an optical image on said scale at a distance s from said reference mark, ds/dn being the rate of change in said distance s with said refractive index n, and d2s/dn2 being the nonlinearity of said rate of change in combination with: stationary refracting means positioned between said entrance face and said reticle, said stationary refracting means deviating said bundles of light by angles of deviation x, dx/dn being the rate of change of said angles of deviation with said refractive index n to be measured, said stationary refracting means being oriented so that dx/dn decreases as n increases thereby causing a reduction of said nonlinearity d2s/dn2, said stationary refractiNg means being constituted by an exit face of said measuring prism, said exit face producing a deviation of said bundles of light in the same direction as the deviation of the bundles of light at said entrance face.
 2. A refractometer according to claim 1 wherein said measuring prism has a refractive index below 1.62 and a prism refracting angle larger than 90*.
 3. A refractometer according to claim 1 wherein said measuring prism has a refractive index above 1.62 and a prism refracting angle larger than 75*.
 4. A refractometer according to claim 1 wherein said measuring prism is composed of at least two components having different optical dispersion.
 5. A refractometer according to claim 4 wherein said components have substantially the same refractive index for one line of the spectrum and are separated by curved faces.
 6. A refractometer operating with a source emitting bundles of light and having a measuring prism with an entrance face to receive different substances within a range of characteristics to be measured corresponding to refractive indices n, an objective having an image plane, a reticle disposed in said image plane and carrying a scale having a reference mark, said entrance face and said reticle being positioned in such a way that bundles of light from said source are refracted and deviated at said entrance face and directed to said objective, and focused by said objective to an optical image on said scale at a distance s from said reference mark, ds/dn being the rate of change of said distance s with said refractive index n, and d2s/dn2 being the nonlinearity of said rate of change in combination with; stationary refracting means positioned between said entrance face and said reticle, said stationary refracting means deviating said bundles of light by angles of deviation x, dx/dn being the rate of change of said angles of deviation with said refractive index n to be measured, said stationary refracting means being oriented so that dx/dn decreases as n increases thereby causing a reduction of said nonlinearity d2s/dn2, said stationary refracting means including a fixed prism having refracting faces and being positioned between said measuring prism and said reticle.
 7. A refractometer according to claim 6 wherein said fixed prism is oriented so that the angle of incidence of said bundles of light at at least one of said faces is positive and comprised between 40* and 75*.
 8. A refractometer according to claim 6 wherein said fixed prism is oriented so that the angle of incidence of said bundles of light at at least one of said faces is negative and comprised between 20* and 50*.
 9. A refractometer operating with a source emitting bundles of light and having a measuring prism with an entrance face to receive different substances within a range of characteristics to be measured corresponding to refractive indices n, said refractive indices varying with temperature changes, an objective having an image plane, a reticle disposed in said image plane and carrying a reference mark, said entrance face and said reticle being positioned in such a way that bundles of light from said source are refracted and deviated at said entrance face and directed to said objective, and focused by said objective to an optical image on said reticle at a distance s from said reference mark, ds/dn being the rate of change of said distance s with said refractive index n, and d2s/dn2 being the nonlinearity of said rate of change in combination with: temperature compensating means compensating for the temperature error resulting from the temperature coefficient of the refractive index of said different substances to be measured, said temperature compensating means comprising a thermal actuator to move one of said objective and said reticle so aS to cause displacement between said optical image and said reference mark in accordance with temperature changes, and stationary refracting means positioned between said entrance face and said reticle, said stationary refracting means deviating said bundles of light by angles of deviation x, dx/dn being the rate of change of said angles of deviation with said refractive index n to be measured, and being oriented so that dx/dn decreases as n increases thereby causing a reduction of said nonlinearity d2s/dn2, the stationary refracting means being constituted by an exit face of said measuring prism, said exit face producing a deviation of said bundles of light in the same direction as the deviation of the bundles of light at said entrance face.
 10. A refractometer according to claim 9 wherein said measuring prism has a refractive index below 1.62 and a prism refracting angle larger than 90*.
 11. A refractor according to claim 9 wherein said measuring prism has a refractive index above 1.62 and a prism refracting angle larger than 75*.
 12. A refractometer according to claim 9 wherein said measuring prism is composed of at least two components having different optical dispersion.
 13. A refractometer according to claim 9 wherein said components have substantially the same refractive index for one line of the spectrum and are separated by curved faces.
 14. A refractometer operating with a source emitting bundles of light and having a measuring prism with an entrance face to receive different substances within a range of characteristics to be measured corresponding to refractive index n, said refractive indices varying with temperature changes, an objective having an image plane, a reticle disposed in said image plane and carrying a reference mark, said entrance face and said reticle being positioned in such a way that bundles of light from said source are refracted and deviated at said entrance face and directed to said objective, and focused by said objective to an optical image on said reticle at a distance s from said reference mark, ds/dn being the rate of change of said distance s with said refractive index n, and d2s/dn2 being the nonlinearity of said rate of change in combination with: temperature compensating means compensating for the temperature error resulting from the temperature coefficient of the refractive index of said different substances to be measured, said temperature compensating means comprising a thermal actuator to move one of said objective and said reticle so as to cause displacement between said optical image and said reference mark in accordance with temperature changes, and stationary refracting means positioned between said entrance face and said reticle, said stationary means deviating said bundles of light by angles of deviation x, dx/dn being the rate of change of said angles of deviation with said refractive index n to be measured, and being oriented so that dx/dn decreases as n increases thereby causing a reduction of said nonlinearity d2s/dn2, the stationary refracting means including a fixed prism having refracting faces and being positioned between said measuring prism and said reticle.
 15. A refractometer according to claim 14 wherein said fixed prism is oriented so that the angle of incidence of said bundles of light at at least one of said faces is positive and comprised between 40* and 75*.
 16. A refractometer according to claim 14 wherein said fixed prism is oriented so that the angle of incidence of said bundles of light at at least one of said faces is negative and comprised between 20* and 50*. 